This invention relates to active matrix electroluminescent display devices comprising an array of electroluminescent display pixels.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional III-V semiconductor compounds. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of an electroluminescent material, for example a semiconducting conjugated polymer, sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer. The polymer material can be fabricated using a CVD process, or simply by a spin coating technique using a solution of a soluble conjugated polymer.
Organic electroluminescent materials exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore be used in passive type displays.
However, the invention is concerned with active matrix display devices, with each pixel comprising an electroluminescent (EL) display element and a switching device for controlling the current through the display elements. Examples of an active matrix electroluminescent display are described in EP-A-0653741 and EP-A-0717446. Unlike active matrix liquid crystal display devices in which the display elements are capacitive and therefore take virtually no current and allow a drive signal voltage to be stored on the capacitance for the whole frame period, the electroluminescent display elements need to continuously pass current to generate light. A driving device of a pixel, usually comprising a TFT (thin film transistor), is responsible for controlling the current through the display element. The brightness of the display element is dependent on the current flowing through it. During an address period for a pixel, a drive (data) signal determining the required output from the display element is applied to the pixel and stored on a storage capacitor which is connected to, and controls the operation of, the current controlling drive device with the voltage stored on the capacitor serving to maintain operation of the switching device in supplying current through the display element during the period, corresponding to a frame period, until the pixel is addressed again.
A problem with known organic electroluminescent materials, particularly polymer materials, is that they exhibit poor stability and suffer ageing effects whereby the light output for a given driven current is reduced over a period of time of operation. While in certain applications such ageing effects may not be critical, the consequences in a pixellated display can be serious as any slight variations in light output from pixels can easily be perceived by a viewer.
It is an object of the present invention to provide an active matrix electroluminescent display device in which this problem is overcome at least to an extent.
According to the present invention there is provided an active matrix electroluminescent display device comprising an array of display pixels each comprising an electroluminescent display element and a driving device for controlling the current through the display element in a drive period based on a drive signal applied to the pixel during an address period preceding the drive period and stored as a voltage on a storage capacitance connected to the driving device, which is characterised in that each pixel includes feedback adjustment means responsive to the potential difference across the display element in the drive period and arranged to adjust the voltage stored on the capacitance in the address period in accordance therewith.
It has been recognised that as the EL display element degrades over time its impedance increases and the potential difference between its anode and cathode increases. The value of the change in potential difference provides a reasonable indication of the state of the element in terms of its light emission/drive current characteristics. Thus, by adjusting the signal voltage stored on the storage capacitance, which determines the display element drive current following addressing, according to the potential difference across the display element which is indicative of the light output characteristic of the display element and provides effectively a positive feedback variable, appropriate compensation for the effects of ageing of the display elements can be made in the driving of the element so that a desired light output level for a given applied drive signal is maintained regardless of possible variations in the drive current level/light output level characteristics of individual display elements in the array.
Although the invention is particularly beneficial in devices whose display elements are polymer LED materials, it can of course be applied to advantage in any electroluminescent device in which the electroluminescent material similarly suffers ageing effects resulting in a lowering of light output levels for a given drive current over a period of time of operation.
A switching device is preferably included in the feedback adjustment means that is operable to prevent current flowing through the display element in the address period and allow current to pass therethrough in the subsequent drive period. This switching device ensures that the potential across the display element at the end of the address period and at the beginning of the drive period is at a known level, i.e. 0 volts, and that the drive signal storage on the storage capacitance is not affected by any drive currents which might otherwise flow through the display element at this time.
In a preferred embodiment, the feedback adjustment means is responsive to the transient potential difference increase across the display element at the beginning of the drive period. Conveniently, a high pass filter circuit connected to the display element and responsive to the rise in voltage thereacross to provide an output in accordance therewith and which controls adjustment of the stored voltage may be used for this purpose. This circuit may include a further switching device operable by the output to connect a source of predetermined potential to the storage capacitance to provide supplemental charging.